Method and a device for charging a battery

ABSTRACT

A device for charging a battery comprises an alternating current source, a rectifier being connected with its input to the alternating current source and being connected with its output to the battery for charging thereof. It also has a member for measuring the charging current from the rectifier to the battery and a means ( 13 ) for comparing the measured charging current with a current limit value and an arrangement adapted to control the voltage on the output of the rectifier based on information about said comparison and decrease it if the measured current exceeds the current limit value for reduction of the charging current. A unit ( 23, 28, 30 ) is adapted to form the current limit value by comparing a current value formed by the value of said measured charging current with a restriction on its positive rise speed with a predetermined maximum current limit value, and to determine the lowest of the so obtained current value and said maximum current limit value as said current limit value for the comparison of said means ( 13 ).

FIELD OF THE INVENTION AND PRIOR ART

[0001] The present invention is related to a device for charging abattery comprising an alternating current source, a rectifier beingconnected with its input to the alternating current source and beingconnected with its output to the battery for charging thereof, thealternating current source being adapted to feed at least one consumerother than the battery with electric energy, a member for measuring thecharging current from the rectifier to the battery, a means forcomparing the measured charging current with a current limit value andan arrangement adapted to control the voltage on the output of therectifier based on information about said comparison and decrease it ifthe measured current exceeds the current limit value for reduction ofthe charging current, and a method according to the preamble of theappended independent method claim.

[0002] First of all, it is pointed out that “the alternating currentsource” herein is to be given a very broad sense and may for instance bean alternating current net, but also be formed by converting electricenergy from a direct current supplying arrangement, such as a directvoltage net, into an alternating voltage, so that the “alternatingcurrent source” is used as a summary for all types of configurations,which are adapted to supply an alternating current on the input of saidrectifier.

[0003] A device of this kind may be used for charging all kinds ofbatteries, but to clearly illustrate the problems the invention tries tosolve, such a device for charging a battery onboard a railway vehiclefor electrified railway traffic will be explained hereinafter, but thusnot at all for the purpose of limiting the invention. Such a batteryonboard a railway vehicle is in most cases used especially to give anextra energy addition when starting the vehicle, but also to feed forexample computers and lighting equipment in the vehicle. The alternatingcurrent consumer, which is fed in parallel with the battery, may be forinstance fans, main compressors, lighting equipment etc in the vehicleor cars connected thereto. The electric energy is normally obtained bythe vehicle taking, from a contact line 1 (see FIG. 1), electric energy,for instance in the form of an alternating voltage, which in Swedennormally is a single phase alternating voltage with a frequency of 16⅔Hz, and that this via a rectifier not shown is converted into a directvoltage, of for instance about 750 V, which is then brought to aconverter in the form of an inverter 2, where it is converted into analternating voltage. The level of this alternating voltage is thenchanged by a transformer 3, from which the alternating voltage is fedfurther to consumers 4 of the kind mentioned. At the same time, a partof the alternating voltage is taken into a converter in the form of arectifier 5, which rectifies the voltage and, at a higher voltage on itsoutput 6 than the voltage of a battery 7, sends a charging currentthereto for charging thereof. Said converters are preferably formed bycurrent valves connected in series, each having at least one switch inthe form of a semiconductor element and a rectifying diode connected inanti-parallel therewith controlled in a conventional manner according toa pulse width modulation pattern as in the embodiment shown in FIG. 2 oras non-controlled rectifiers, e.g. diode rectifiers, as in theembodiment according to FIG. 1.

[0004] From the contact line 1, the engine of the vehicle obtainselectric energy also in parallel with the shown device, preferably by anot shown converter.

[0005] The charging current to the battery can be measured byschematically indicated members 8, whereas the voltage of the batterycan be measured by members 9. The measurement values obtained in thismanner are used for controlling the output voltage of the rectifier 5and thereby the charging current to the battery 7. When the outputvoltage on the output 6 exceeds the voltage of the battery, the batterywill be charged by the coil 10. The device also has an arrangement 11adapted to control the voltage on the output of the rectifier based oninformation about a comparison of the measured charging current with acurrent limit value and decrease it if the measured current exceeds thecurrent limit value for reduction of the charging current. This takesplace at a device of the kind shown in FIG. 1 by controlling theinverter 2 so that a suitable voltage level is obtained on its output.However, this has the disadvantage that when the voltage to therectifier 5 has to be decreased, the level of the voltage to theconsumers 4 will then be reduced automatically.

[0006]FIG. 2 shows a variant of the device according to FIG. 1, at whichsaid arrangement 11 controls the rectifier instead, so that the feed tothe battery and that to the consumers practically become independent ofeach other.

[0007] In FIG. 3 it is shown somewhat more in detail how the arrangement11 of a device of this kind may function. The value of the measuredcharging current arrives at 12 and its negative value is added in anaddition member 13 to a current limit value arriving at 14. If theaddition in 13, which in practice is a comparison of the chargingcurrent with a current limit value, which the charging current shouldnot exceed to prevent triggering of overcurrent protection and stoppingthe operation of the entire device according to FIGS. 1 and 2 withinterruption of the feed of electric energy to said consumers 4 as wellas to the battery as a result, gives a negative value on the input 15 toa member 16, a reference voltage value for the voltage on the output 6of the rectifier is output on the output of said member, and is thenmade lower than a predetermined voltage value input to the member 16 at17. However, should the value of the charging current be lower than saidcurrent limit value, then the reference voltage input at 17 would alsobe output at 18. At 19, the battery voltage multiplied by −1 is input toan addition member 20, which inputs the result of the addition to amember 21, which at a lower reference voltage than battery voltage sendssignals to a control unit 22, which in the device according to FIG. 1controls the inverter 2 as concerns voltage peak value and in the deviceaccording to FIG. 2 controls the rectifier 5 as concerns voltage peakvalue. If the battery voltage is higher than the reference voltage at18, said voltage peak value and thereby the voltage out of the rectifierand thereby finally the charging current to the battery are reduced.

[0008] In FIG. 4 it is schematically shown what could happen if thelevel of the voltage into the rectifier suddenly decreases, for instancedue to a large power output from a consumer when starting the same. Itis schematically illustrated how the output voltage U₆ varies over time,U_(B) being the battery voltage. Furthermore, the charging current I_(B)is drawn and said current limit value I_(B),lim is drawn with a brokenline over time. It is apparent that when the output voltage from therectifier decreases below the battery voltage, the charging current islowered to 0, but when said output voltage then rises again above thebattery voltage, the result due to the low internal resistance in thebattery will be a very steep rise of the charging current, which oftenleads to a very large overshoot of the charging current. This impliesthat the charging current passes the current limit value and thearrangement 11 performs a control in the manner shown in FIG. 3 to lowerthe charging current below this value. However, the current overshoot isat times so large that over-current protection manages to trigger toprevent damages to transformers and diodes, so that the entire device isshut off with significant problems as a result. It is pointed out thatvoltage drops with corresponding problems as a result also can arise onthe load side of the battery, where also large loads may occur.

SUMMARY OF THE INVENTION

[0009] The object of the present invention is to provide a device and amethod of the initially defined kind, which find a remedy for the abovementioned problems with risk for triggering over-current protection uponsuddenly rising output voltage from the rectifier. This object isobtained according to the invention by providing such a device with aunit adapted to form said current limit value by comparing a currentvalue formed by the value of said measured charging current with arestriction on its positive rise speed with a predetermined maximumcurrent limit value, and to determine the lowest of the so obtainedcurrent value and said maximum current limit value as said current limitvalue for the comparison of said means.

[0010] By dynamically varying the current limit value in this manneralready when the charging current starts to rise, even if the currentintensity then lies far below the value, which can really be accepted,the charging current will pass said current limit value if it risesfaster than said positive rise speed and thereby the arrangement willcontrol the voltage on the output of the rectifier to decrease forreduction of the charging current, so that the current intensityrequired for the over-current protection to be triggered is neverreached. Accordingly, the invention is not based on that the regulationof the charging current is done faster, but it is started earlierinstead, i.e. far below the maximally allowed level on the current. Thisway of regulation implies a very smooth way of restricting transients.

[0011] According to a preferred embodiment of the invention said unitcomprises members adapted to add a maximum current value addition/unitof time to the value of the measured charging current and membersadapted to compare a so formed current value with existing chargingcurrent and determine the lowest of them as a theoretical chargingcurrent value and send this to said adding members so as there to beconsidered as a new charging current value, and that the comparingmember is adapted to send said theoretical charging current values to amember for comparing that with said maximum current limit value todetermine the lowest of these as current limit value for the comparisonof said means. Hereby it is ensured that the current limit value for thecomparison of said means, which is the basis for the actual control ofthe output voltage from the rectifier, on one hand never gets higherthan the value of the sum of the charging current and the maximumcurrent value addition, which preferably at normal charging currentimplies that this sum becomes considerably lower than the maximallyallowed current limit value, and on the other hand that the currentlimit value cannot rise too quickly. In practice, this implies that thecurrent limit value will be exceeded by the charging current directlywhen it starts to rise faster than the positive rise speed allowed bysaid restriction, and this irrespective of the level of the chargingcurrent at that time, so that the regulation of the output voltagedownwards can be started already at very low, but quickly risingcharging currents.

[0012] According to another preferred embodiment of the invention thedevice is adapted to be a part of the electric system of a railwayvehicle, and the alternating current source is adapted to obtainelectric energy from a contact line for said railway vehicle. Thiscontact line could be a direct current line as well as an alternatingcurrent line. The device is especially well suited for this application,as the problem which the invention tries to solve often appears in theelectric system of such railway vehicles provided with devices forcharging batteries.

[0013] The invention is also related to a method for charging a batteryaccording to the appended method claims. The advantages with such amethod are clearly apparent from the discussion above concerning thedevice according to the invention.

[0014] Further advantages with and advantageous features of theinvention are apparent from the following description and the otherdependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Below follows a description of the device according to apreferred embodiment of the invention with reference to appendeddrawings, in which:

[0016]FIG. 1 is a schematical view illustrating a device for charging abattery according to a first type, on which the invention may beapplied,

[0017]FIG. 2 is a schematical view illustrating a device for charging abattery according to a second type, on which the invention may beapplied,

[0018]FIG. 3 illustrates more in detail a part of the device accordingto prior art,

[0019]FIG. 4 is a diagram over the output voltage U₆ from the rectifier,the battery voltage UB and the charging current I_(B) over time t forthe previously known device according to FIG. 3,

[0020]FIG. 5 is a view of a part of a device according to a preferredembodiment of the invention,

[0021]FIG. 6 is a diagram illustrating how a current limit valueI_(B),lim obtained by the device according to FIG. 5 changes over time tand depending on the charging current I_(B), and

[0022]FIG. 7 is a diagram corresponding to FIG. 4 for the deviceaccording to the invention according to FIG. 5.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0023] Actually, it can be considered as if two preferred embodiments ofthe invention are described hereinafter, as the part of the deviceaccording to the invention, which is shown in FIG. 5 can be consideredto be part of a device according to FIG. 1 as well as FIG. 2 as a partof the arrangement 11 shown there for direct or indirect control of thevoltage on the output of the rectifier.

[0024] This arrangement first of all comprises a member 23, upstream ofthe means 13 in FIG. 3, adapted to, on its input 24, receive a value ofthe charging current to the battery 7 delivered from a member 8 (seeFIGS. 1 and 2). This member 23 is adapted to make a comparison of thisvalue with a value arrived at a second input 25 and send the lowest ofthese values further on its output 26 to an input 27 of a member 28,which adds to this value a predetermined addition Δ input on anotherinput 29. Only a certain addition per unit of time can be added to thevalue arriving at 27. A is substantially smaller than, preferablysmaller than 20% of the normal difference between I_(B) and themaximally allowed current limit value. The so obtained value is sentback to the input 25 of the member 23 to be compared with the chargingcurrent. Since the lowest of the charging current value and the value onthe input 25 is continuously sent further to the member 28 which gives acertain addition per unit of time thereto, the value on the output 33 ofthe member 28 cannot increase faster than with this addition per unit oftime. Simplified, it could be expressed as if I_(B)+Δ arrives at 27 anda certain time has not passed since making the last addition of Δ, noaddition at all takes place in 28. This value is then sent further to amember 30, where it is compared with a maximum current limit valueobtained on an input 31 and the lowest of these two values is sentfurther to the means 13 illustrated in FIG. 3, which works in the sameway as described with reference to FIG. 3. After the means 13 the othermembers 16, 20, 21 and 22 shown in FIG. 3 follow, having the samefunction as previously described.

[0025] In FIG. 6 it is illustrated how the value I_(B),lim on the output33 from the member 28 will vary over time upon changes of the value ofthe charging current obtained on the input 24 of the member 23. Thus,the result of the members 23 and 28 with the interconnection thereof isshown. It is apparent that I_(B),lim will always be Δ larger than I_(B)as long as the value of the charging current does not rise too steeply,i.e. faster than the addition Δ per unit of time. Similarly, the fallspeed of I_(B),lim is not restricted. When contrary thereto the value ofthe charging current rises faster than the rise speed corresponding tothe maximum addition Δ per unit of time in the member 28, the value ofthe charging current will exceed I_(B),lim and thereby a negative valuewill be achieved in 13 and the output voltage from the rectifier will beregulated downwards. However, this is of course always made when thecharging current exceeds the maximum current limit value delivered onthe input 31 at the latest.

[0026] In FIG. 7 it is shown what will happen to the current I_(B) inpractice when the output voltage U₆ from the rectifier varies in the waypreviously illustrated in FIG. 4 for the previously known device in caseof a device according to the invention according to FIG. 5. It isapparent that the quick rise of I_(B), which normally would havefollowed as of the time to, will not take place, as the charging currentdirectly will rise more quickly than the quickest possible rise of thedynamic limit value delivered by the member 28 and thereby a regulationof the voltage U₆ downwards will start. Therefore, the value of thecharging current does not reach up to the maximum current limit valueindicated by the broken line 32 and no over-current protection has to betriggered.

[0027] The invention is of course not in any way restricted to thepreferred embodiment described above, but many possibilities tomodifications thereof will be apparent to a person with ordinary skillin the art without departing from the scope of the invention, such asthis is defined by the appended claims.

[0028] Other variants than the one shown above for obtaining a dynamiccurrent limit value with restricted rise speed are conceivable, and allof these functions could of course be comprised in one single unit, andthe same holds for the entire arrangement mentioned above including thefunction of calculating a dynamic current limit value, which could beformed by a computer unit programmed in a suitable manner. The inventionis not restricted to the described, time discrete implementation, butalso implementation continuous in time, for instance with analoguetechnology, would be conceivable.

[0029] It is also within the scope of the invention to design therestriction of the rise speed of the current limit value in such amanner that this is dependent on some parameter, for instance the sizeof the charging current, if this should be desired for some particularapplication. For instance, a higher rise speed of the charging currentcould then be allowed within some value range thereof than within anyother without the regulation of the output voltage from the rectifierinterfering.

1. A method for charging a battery (7) via a rectifier (5) beingconnected with its input to an alternating current source (1, 2) adaptedto provide at least one consumer (4) other than the battery withelectric energy and being connected with its output (6) to the battery,in which the charging current from the rectifier to the battery ismeasured, the measured current is compared with a current limit valueand if the measured current exceeds this current limit value, the outputvoltage from the rectifier is controlled to decrease for reduction ofthe charging current to the battery, characterized in that said currentlimit value is determined continuously by comparing a predeterminedmaximum current limit value with a current value formed by the value ofsaid measured charging current with a restriction on its positive risespeed, and the lowest of said predetermined maximum current limit valueand said formed current value is chosen as current limit value for saidcomparison with the measured charging current.
 2. A method according toclaim 1, characterized in that a maximum current value addition/unit oftime is added to the value of the measured charging current, the soformed current value is compared with existing charging current and thelowest of them is determined as a theoretical charging current valuewhich subsequently is subjected to said addition so as there to beconsidered as a new charging current value, and said theoreticalcharging current values are compared with said maximum current limitvalue and the lowest of these is determined as said current limit value.3. A method according to claim 1 or 2, characterized in that electricenergy is fed to the alternating current source via a contact line (1 )for railway vehicles.
 4. A method according to any of the precedingclaims, characterized in that a direct voltage is converted into analternating voltage in an inverter (2) for feeding to the input of saidrectifier (5).
 5. A method according to claim 4, characterized in thatthe output voltage on the output (6) of the rectifier is controlled bycontrolling said inverter (2).
 6. A method according to any of claims1-4, characterized in that the output voltage on the output of therectifier is controlled by controlling the rectifier (5).
 7. A devicefor charging a battery (7) comprising an alternating current source (1,2), a rectifier (5) being connected with its input to the alternatingcurrent source and being connected with its output (6) to the batteryfor charging thereof, the alternating current source being adapted tofeed at least one consumer (4) other than the battery with electricenergy, a member (8) for measuring the charging current from therectifier to the battery, a means (13) for comparing the measuredcharging current with a current limit value and an arrangement (11)adapted to control the voltage on the output of the rectifier based oninformation about said comparison and decrease it if the measuredcurrent exceeds the current limit value for reduction of the chargingcurrent, characterized in that it comprises a unit (23, 28, 30) adaptedto form said current limit value by comparing a current value formed bythe value of said measured charging current with a restriction on itspositive rise speed with a predetermined maximum current limit value,and to determine the lowest of the so obtained current value and saidmaximum current limit value as said current limit value for thecomparison of said means.
 8. A device according to claim 7,characterized in that said unit comprises members (28) adapted to add amaximum current limit value addition/unit of time to the value of themeasured charging current and members (23) adapted to compare a soformed current value with existing charging current and determine thelowest of them as a theoretical charging current value and send this tosaid adding members (28) so as there to be considered as a new chargingcurrent value, and that the comparing member is adapted to send saidtheoretical charging current values to a member (30) for comparing thatwith said maximum current limit value to determine the lowest of theseas current limit value for the comparison of said means (13).
 9. Adevice according to claim 7 or 8, characterized in that it is adapted tobe a part of the electric system of a railway vehicle, and that thealternating current source is adapted to obtain electric energy from acontact line (1) for said railway vehicle.
 10. A device according toclaim 9, characterized in that said battery (7) is adapted to be usedfor supplying electric energy to an engine of the vehicle when startingthe vehicle.
 11. A device according to claim 9 or 10, characterized inthat said consumer (4) is one or more of fans, main compressors, andlighting equipment included in the vehicle.
 12. A device according toany of claims 9-11, characterized in that said consumer (4) is connectedin parallel with the battery (7).
 13. A device according to any ofclaims 7-12, characterized in that it comprises an inverter (2) adaptedto convert a direct voltage into an alternating voltage for feeding theinput of said rectifier (5).
 14. A device according to claim 13,characterized in that a transformer (3) for changing the level of thevoltage from the inverter (2) to said consumer (4) and the battery (7)is arranged between the inverter and the battery and the consumer,respectively.
 15. A device according to claim 13 or 14, characterized inthat said arrangement (11) is adapted to control the inverter (2) forcontrolling the output voltage on the output (6) of the rectifier.
 16. Adevice according to any of claims 7-14, characterized in that saidarrangement (11) is adapted to control the rectifier (5) for controllingthe output voltage on the output (6) of the rectifier.